Bug 46576: Use HTML for images

Follows the dom spec at:

http://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec#Images

This patch introduces a bunch of extra test failures. All of these are related
to us no longer preserving extra captions on round-trip. In production this
won't matter since selective serialization will hide this.

Bug: 46576
Change-Id: I6d976263775415fabeddb767120f61a48ce3632f